Foam supply systems of the above-indicated type are known in the art by the term CAFS (Compressed Air Foam System) and WEPS (Water Expansion Pumping System). A typical system includes a foam injector system, a water pumping system, and an air system including an air compressor for supplying air under pressure. For example, when employing mixture ratios of 1 CFM of air to 1 GPM of water, these systems can produce very desirable results in fire fighting by the use of "Class A" or "Class B" foams to help achieve fire suppression and to deal with increased fire loads and related hazards.
Compressed air foam systems of the indicated type also generally include a device, such as a venturi, whereat the compressed air and foam solution are combined and an arrangement whereby the combined flow is delivered through a mixing device to a fire stream delivery means. The purpose of the mixer is to create a mixing action to produce a foam as the mixture of foam solution and air flow from the venturi to the upstream end of the fire delivery means. The mixer is particularly useful when the fire stream delivery means is a deck gun arrangement, in which case there is a minimum length of pipe between the venturi means and the inlet to the fire stream delivery means. Mixers of this type are known in the art as stationary or motionless mixers and function to enhance mixing by adding turbulence to the flow while keeping the pressure loss as low as possible.
One compressed air foam system of the above-indicated type is disclosed in U.S. Pat. No. 5,255,747, assigned to Hale Fire Pump Company. The mixer employed in the system disclosed in said patent comprises a stationary mixer which is provided with a plurality of vanes which function to create turbulence without an excessive pressure drop as the mixture of foam solution and air flow from a venturi to the upstream end of a fire hose.
Mixers of the above-described type are also disclosed in U.S. Pat. Nos. 3,923,288 and 4,034,965. These patents disclose a vane type of mixer similar to that disclosed in U.S. Pat. No. 5,255,747. The mixing vanes of the mixer disclosed in these patents are constructed and arranged so that the vanes overlap to provide a plurality of axially overlapping regions which provide a mixing matrix introducing complex velocity vectors onto the materials being mixed, the design being such that the material flowing through the conduit defined by the mixer is directed through a substantial radial displacement over a short distance, each main element imparting a rotational vector to the material stream which is then transformed to a lateral or radial vector. U.S. Pat. No. 4,614,440 discloses a stationary mixer of the indicated type which comprises a mixing apparatus in the shape of a conduit made up of individual biscuit sections, each of which contains a plurality of openings therethrough containing mixing elements which induce a rotational angular velocity to the fluid stream.
The main disadvantage of the mixers in use today, which are typified by the prior art mixers discussed above, is that while a good mixing of the fluids is achieved, this is achieved at the expense of a substantial pressure drop. Another disadvantage is the undue complexity in the design of the prior art mixers.